Historically anaerobic sludge digestion has been used for stabilization of primary clarifier sludges. More recently, it has been applied to biological sludges produced by the activated sludge or trickling filter processes, and sludge mixtures containing significant industrial waste contributions.
Anaerobic digestion involves a complex microbiological community in which various groups of organisms carry out initial, intermediate and final stabilization steps. The groups of organisms work together in a symbiotic relationship. Not only is a constant temperature important but the food supply should be available on a steady state basis. This requires that the mass be mixed on a steady state basis.
Mixing is more than moving masses of material around relative to the containment vessel in which the anaerobic digestion takes place. Mixing must provide differential displacement of masses relative to each other. As the displaced units of mass become smaller, mixing becomes more effective. Ideal process kinetics and rates of reaction can only be approached when the food for each bacterium is evenly distributed throughout the digesting mass. The end products of the hydrolyzing bacteria become the food for the organic acid producing bacteria. The end products of one group of anaerobic bacteria become the food for the next group in the overall sequence. It continues all the way to the final steps that produce stable end products and gas, largely methane.
In the United States the containment vessel usually had a nearly flat bottom cylindrical side wall and a slightly domed roof because this shape was least expensive to build. The vessel shape has not been considered an important or critical factor by American engineers. Instead of relating digester mixing problems to both vessel shape and the mixing devices, American engineers and process specialists have concentrated on the mixing devices.
Early American mixing systems concentrated on the use of gas injection to induce mixing. The naturally occurring digester gas was compressed mechanically and injected into the bottom zone of the digester with bottom supported diffusers or roof supported gas diffuser lances. Such gas compression systems are complex and create maintenance problems. Process performance is erratic and unpredictable. As scientific investigation continued, systems using draft tube pumps became the mixing systems of choice. Most of the draft tube pumps utilized compressed gas injected into the draft tube interior near the bottom. Some manufacturers utilized propeller pumps located near the top of the draft tube to provide the pumping action. The American draft tube mixing systems have concentrated on large volume displacement of digesting sludge from the bottom of the digester to the top.
While the fundamental anaerobic digestion principles are sound, several problems continue to cause erratic and unpredictable anaerobic digester behavior in America. The large diameter-to-depth ratio tanks with nearly flat bottoms common in America makes them difficult to keep completely mixed. In the last 15 years, the amount of mixing energy utilized has increased substantially. With increased mixing overall digester performance has improved but still is erratic and unpredictable.
The gas induced mixing systems have special problems. The gas mixing systems encourage foam and lighter than water materials to accumulate at the surface. As the surface foam layer increases it begins to develop some strength because of drying caused by the migrating gas.- As these surface layers increase in depth, they reduce the active volume of the digester. The reduced volume contributes to the eventual overload and failure of the process.
Some manufacturers use propeller pumps to institute mass pumpage from the digester vessel bottom to the top. The propeller pump avoids some of the gas lift pump problems but has its own problems. The propeller pumps have low energy efficiency. The propellers and shafts are subjected to a severe environment. The sludge has high corrosion and erosion potential. Stringy materials can collect on the blades and unbalance the system causing mechanical failures.
In summary the large mass displacement mixing systems used in conventional American digesters do not create optimum mixing conditions.
In Europe, particularly Germany, more importance has been associated with true and effective mixing in anaerobic digesters. German digesters generally have a different vessel shape. While the vessel shapes vary some, the basic conditions stay the same. All of the vessels have a steep-sided bottom cone with a barrel, sphere or cylinder middle zone and an inverted cone on top. This basic shape, including its elliptical variations, makes it easier to prevent solids from settling or floating out of the main active mass. The German process performance is quite stable and consistent.
Most of the German facilities use pumped liquid circulation for mixing. The smaller facilities have systems that rely on selective displacement to move mass from various digester locations and reinject it to create circulation. These pumped circulation systems have a low energy efficiency. The larger facilities combine a central draft tube pump with externally pumped circulation to provide mass movement. The vertical mass movement of mixing together, coupled with the natural forces of thermal and gas upward migration, work to provide interference mixing. The draft tube pumps are activated by a propeller pump at the draft tube top. Through very conservative design, the propeller pumps are mechanically reliable. However, the propeller pumps are extremely expensive and are not particularly energy efficient. The German vessel shape allows the mixing pumpage rate to be considerably less than necessary with American systems. The energy required by German digester systems is substantially less.
From the above discussion it is clear that new apparatus and methods are needed which will achieve increased mixing of the liquid volume in an anaerobic digester, desirably with reduced energy consumption and which involves equipment which is relatively simple to construct and operate and which is resistant to the harsh conditions present in such a digester.